Quality Testing
Delivering adequate and consistent voice quality is a stumbling block for wired VoIP, and QoS is even more challenging in a Wi-Fi environment.
SpectraLink tackles QoS and timing problems with its SVP, which also addresses call-admission control. Although the company doesn't make APs, it has worked closely with wireless infrastructure vendors to build SVP support into APs and wireless controllers, including those from Cisco. SpectraLink's market share--and the fact that it shares customers with all major WLAN infrastructure vendors--helped accomplish this.
Working within 802.11, SpectraLink does wireless QoS in two ways. First, the AP prioritizes SVP-tagged traffic over data traffic. If the AP supports multiple queues, a high-priority queue is used for voice packets. If the AP has only one queue, voice packets are put at the head of the line. Second, SVP gains priority access to the medium by using a back-off value of zero, rather than a random value between 0 and, typically, 31. So while other devices will average back-off values around 16, SVP packets and SpectraLink handsets will have first crack at the medium.
Rivals, including Vocera, label SVP "cheating," but we have to concede that it's an effective means, within 802.11, to assure wireless QoS. Would we prefer a more standard method? Sure. Fortunately, in September 2004, the Wi-Fi Alliance introduced the WMM (Wireless Multimedia Extensions) certification based on the 802.11e QoS standard, which the IEEE ratified in September 2005. With the appropriate WMM support, APs can assign wireless-bound traffic one of four priority levels: voice, video, best effort and background; applications and clients can assign priority levels as well, with appropriate drivers. WMM requires support on the client and AP to work effectively.
Although not as aggressive as the SpectraLink zero back-off, each priority level has an appropriately sized upper-limit. Unfortunately, even though WMM was introduced more than two years ago, enterprise Vo-Fi vendors have been slow to build it into their products. When Cisco last released a software update for its aging 7920, in early 2006, for example, WMM support was conspicuously absent. Instead, Cisco relies on its proprietary CCX and native controller QoS features. Cisco's newest Vo-Fi phone, the 7921G, is WMM-compliant, a welcome change and necessary component of CCXv4.
Vocera recommends creating a separate VLAN for voice and assigning the whole VLAN priority. It says it endorses the WMM as standard but its own product does not support it, and claims that few customers ask for it. SpectraLink supports WMM, but recommended we turn it off in our tests and use SVP, which it says is likely to be more effective! Hitachi is the only vendor that has fully embraced WMM, and our tests demonstrated the positive results; see nwcreports.com for more.
It's a chicken-and-egg issue: Few applications require what WMM delivers, so developers have been slow to add it. WLAN infrastructure vendors, recognizing the potentially small uptake, have focused their development efforts on more pressing projects.
